l2tp: Don't assign net->gen->ptr[] for pppol2tp_net_ops.

l2tp: Don't assign net->gen->ptr[] for pppol2tp_net_ops.

Commit fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and
ppp parts") converted net->gen->ptr[pppol2tp_net_id] in l2tp_ppp.c to
net->gen->ptr[l2tp_net_id] in l2tp_core.c.

Now the leftover wastes one entry of net->gen->ptr[] in each netns.

Let's avoid the unwanted allocation.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>